It depends on how far from the basking spot you are going to put the bulb. Go to the Zoomed website. They'll tell you. I think the 5.0 is only effective at less than 12 or 18 inches, and the 10.0 is effective at up to like 24 inches. Double check. I bought the 10.0 because I'm keeping my bulbs about 14 inches away. I'd rather be safe than sorry. None of these bulbs can even come close to the sun except for the mercury vapor bulbs and other $400 units.

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